1. Field of the Invention
This invention relates to a process for producing a crystal, particularly to a process for producing a crystal to be formed on a light-transmissive substrate.
2. Related Background Art
In the prior art, single crystalline thin films to be used for semiconductor electronic devices, optical devices, etc. have been formed by epitaxial growth on a single crystalline substrate. However, for epitaxial growth of a single crystalline thin film on a single crystalline substrate, it is necessary to take matching in lattice constant and coefficient of thermal expansion between the single crystalline material of the substrate and the epitaxial growth layer, and there has been involved the problem that the kinds of the substrate material were limited to extremely narrow scope for formation of a single crystalline layer capable of being used to prepare a device of good quality.
On the other hand, in recent years, research and development have been actively done about three-dimensional integrated circuits for accomplishing high integration and multi-functionality by forming semiconductor elements by lamination in the direction normal to the substrate, and also research and development about large area semiconductor devices such as solar batteries or switching transistors for liquid crystal picture elements, etc. in which elements are arranged in an array on an inexpensive glass plate are becoming more active year by year.
What is common in these researches and developments, the technique to form a semiconductor film on an amorphous insulating material and form an electronic element such as a transistor, etc. thereon is required. Among them, it has been particularly desired to have a technique to form a single crystalline semiconductor of high quality on an amorphous insulating material.
However, generally speaking, when a thin film is formed on an amorphous insulating substrate such as SiO.sub.2, etc., due to deficiency of long length order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline, whereby it was very difficult to form a single crystalline semiconductor of high quality. Here, the term "amorphous" refers to the state in which short length order to the minimum extent on the order of atom may be maintained, but there is no long length order, while the term "polycrystalline" refers to the state in which single crystal grains having no specific crystal direction are gathered while being separated with grain boundaries.
As a solution of the problems as mentioned above in the prior art, the present inventor has proposed in Japanese Patent Application No. 153273/1986 a method for forming a crystal, which comprises adding on a substrate having a non-nucleation surface (S.sub.NDS) of a small nucleation density at a small area sufficient to effect crystal growth from only a single nucleus at a desired position of the non-nucleation surface (S.sub.NDS) an amorphous material (M.sub.L) for forming a nucleation surface (S.sub.NDL) having a nucleation density (ND.sub.L) greater than the nucleation density (ND.sub.S) of the non-nucleation surface (S.sub.NDS), the amorphous material (M.sub.L) being different from the material (M.sub.S) constituting the non-nucleation surface (S.sub.NDS), to form the nucleation surface (S.sub.NDL), and then applying crystal forming treatment on the substrate to form a single crystal nucleus on the nucleation surface (S.sub.NDL), thereby growing a single crystal from the single nucleus. This crystal forming method shows that it is possible to form a single crystal on the surface of a substrate having a surface made of an insulating amorphous material.
The present inventor has intensively studied about the above method for forming a crystal and the crystal formed thereby, and consequently found by way of various crystal analytical methods that although good crystal can be formed at the upper part of the crystal to be formed, but there is a case where crystal defects occur at the interface between the amorphous surface and the crystal formed. These crystal defects will become the cause for leak current or trap in formation of electronic devices, thus affecting electrical characteristics thereof.